1. Field of the Invention
The present invention relates to photodiodes. In particular, it pertains to avalanche photodiodes with large surface areas made with planar technology.
2. Description of the Prior Art
Previous segmented photodetectors, such as quadrant detectors, were made of silicon. Silicon, an indirect gap semiconductor, has a relatively deep penetration depth. The depth varies for different wavelengths. The depth is 200 .mu.m, for 1.06 .mu.m light. Photocarriers are generated within a volume equal to the surface area times the penetration depth. Therefore, in multi-element silicon detectors the active areas have to be separated by 100-200 .mu.m to limit cross-talk. In some infrared target seeker optics, it is necessary to be able to observe images smaller than the element separation of silicon detectors.
Thus, silicon photodetectors require a trade-off between the possibility of losing the image as it moves across the strip separating the quadrants or sectors of the photodetector and losing the image because of cross-talk causing a false tracking signal.
The penetration depth of 1.06 .mu.m radiation in direct gap semiconductors, such as GaAs.sub.l-x Sb.sub.x, is 1-5 .mu.m. If the direct gap semiconductor material exists as an epitaxial layer on a substrate of wider gap energy, then the substrate material can serve as a filter.